ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
G2I 
where the positive luminosity has such a tortuous course as has been described that 
this phenomenon presents itself. Repeated experiments have demonstrated beyond 
the possibility of doubt that where the positive luminosity lies along the inner surface 
of the tube (as is usually the case) the image of a small object lying over it will be cast 
perpendicularly upon it. Hence the case we are about to discuss must be taken as an 
exceptional case due to the presence of special circumstances, the nature of which it is 
one of our aims to discover, but not in any way casting doubt on the conclusions which 
we have already drawn from observations upon the normal behaviour of the positive 
luminosity and its attendant phosphorescence. 
If the object which is brought into the path of the positive luminosity in its passage 
across the tube be a non-conductor, there is no special peculiarity in the shadow case. 
It will be an ordinary phosphorescent shadow, whose direction is defined in the way 
we have given above. But if it be a conductor, and especially if it be a portion of a 
conductor of some little magnitude such as the end of a piece of wire, it will be found 
that the shadow is bulged out to a very considerable extent. And if the finger be 
placed against the outside of the tube exactly opposite to the other end of the small 
conductor, this bulging out will become immensely increased. 
Now there can be but one possible interpretation of these phenomena. The bulging 
out must be caused (as w r e saw in a previous case) by a discharge of negative electricity 
from the sides of the wire, and such discharge must be in response to a demand for it 
in the tube. But here we come to a very remarkable peculiarity of the present case. 
This demand must be intensely local; for while in the case of relief phosphorescence 
there was no perceptible bulging out of the shadow of a conductor that was partly 
within the range of the streams that were crossing the tube, such a bulging out not 
only occurs in the present case but is a most marked phenomenon. Thus we have 
direct proof that the positive luminosity marks a locus of intense demand for negative 
electricity. 
A very curious variation of this experiment may here be referred to in order to 
strengthen the conclusions just drawn. The zigzag positive luminosity was made to 
cut the thin projecting wire that formed the positive terminal. Its shadow gave no 
sign of bulging out, and behaved as though it was a non-conductor. It is obvious that 
no negative electricity could be drawn from it as a response, and hence there was no 
bulging out of the shadow. To test the matter still further, the metallic object that 
had previously been experimented on was shaken down into contact with the positive 
terminal, and its shadow was observed. It was found to have no bulging, but to be 
thin and sharp like the shadow of a non-conductor. 
We have thus direct evidence of the intense local demand for negative electricity in 
the track marked by the positive luminosity.' 5 ' It seems paradoxical that this can co¬ 
exist with streams of molecules proceeding along it. But it must be remembered that 
we have no evidence that they are (at all events throughout the whole of their course) 
* We shall return to this subject in Section XXVIII. 
4 L 
MDCCCLXXX. 
